As a representative example of a red phosphor, not only a sulfur-containing phosphor having a host material which includes sulfur such as (Ca, Sr)S:Eu, (Zn, Cd)(S, Se):Ag, and Ba2ZnS3:Mn is known but also a red phosphor consisting of calcium sulfide (CaS) as a host material, Eu as a luminescent center (that is, activator), and Mn, Li, Cl, Ce, Gd, or the like as a sensitizing agent (co-activator) is disclosed in Patent Document 1 or 2.
Further, as a green phosphor, a sulfur-based thiogallate phosphor expressed as (Ca, Sr, Ba)(Al, Ga, In)2S4:Eu is known (Patent Documents 3, 4, and 5). Further, as a blue phosphor, a sulfur-containing phosphor such as SrS:Ce, (Sr, Ca)Ga2S4:Ce, BaAl2S4:Eu, and Ba2SiS4:Ce is known.
As described above, a sulfur-containing phosphor having a host material which includes sulfur is excited by an LED or the like and can emit various colors depending on composition, and therefore it is being developed as various phosphors.
Meanwhile, the sulfur-containing phosphor of various kinds described above can easily react with water. As such, when stored or used in an atmospheric condition, a problem arises in that it reacts with moisture or the like in the air, and as a result of hydrolysis, the phosphor is deteriorated to yield weak light emission.
Yet another problem has been noticed that the sulfur in a sulfur-containing phosphor reacts with water to generate hydrogen sulfide gas, and the resulting hydrogen sulfide gas inhibits, when used for a white LED in particular, curing of a silicone resin to be admixed with the phosphor or oxidizes a metal member present inside the device such as Ag coating film that is coated for increasing reflection ratio of a lead frame (hereinafter, referred to as an “Ag reflection film”), yielding lowered reflection performance or providing cause for electrical defects such as disconnection.
In this connection, for purpose of improving moisture resistance or the like, a method of coating the aforementioned kinds of phosphor with a glass material or the like (Patent Documents 6, 7, 8, 9, and 10), a method of coating the surface of phosphor particles by chemical vapor phase reaction method (Patent Document 11), and a method of attaching particles of a metal compound (Patent Document 12) or the like are conventionally suggested.